1. Field of the Invention
The invention disclosed broadly relates to wireless communications and more particularly relates to RF spectrum monitoring for wireless systems.
2. Related Art
The invention disclosed is related to U.S. Pat. No. 6,615,040 to Mathilde Benveniste, issued Sep. 2, 2003, entitled “Self-Configurable Wireless Systems: Spectrum Monitoring In A Layered Configuration”, assigned to the AT&T Corporation, and incorporated herein by reference.
The invention disclosed is also related to U.S. Pat. No. 5,404,574 to Mathilde Benveniste, issued Apr. 4, 1995, entitled “Apparatus And Method For Non-Regular Channel Assignment In Wireless Communication Systems”, assigned to the AT&T Corporation, and incorporated herein by reference.
The invention disclosed is also related to U.S. Pat. No. 5,809,423 to Mathilde Benveniste, issued Sep. 15, 1998, entitled “Adaptive-Dynamic Channel Assignment Organization System And Method”, assigned to Lucent Technologies, Inc., and incorporated herein by reference.
The invention disclosed is also related to U.S. Pat. No. 5,787,352 to Mathilde Benveniste, issued Jul. 28, 1998, entitled “System and method for management of neighbor-channel interference with power control and directed channel assignment”, assigned to the AT&T Corporation, and incorporated herein by reference.
3. Background
The IS-136 Digital Cellular/PCS Standard
An IS-136 Digital Cellular/PCS system includes one or more cellular radio telephones or mobile devices within the communications range of a base station. The base station can have one or more radio transceivers, a control computer, and an antenna system. The base station is connected by means of a mobile switching center (MSC) to the public switched telephone network (PSTN). The mobile switching center provides the functionality of a telephone central office and is additionally responsible for call processing, mobility management, and radio resource management. The IS-136 Digital Cellular/PCS network architecture and system operation are specified by the Telecommunications Industry Association, TIA/EIA Interim Standard: TDMA Cellular/PCS-Radio Interface—Mobile Station-Base Station Compatibility, Revision A, TIA/EIA/IS-136-A, October 1996.
FIG. 1 shows the prior art state diagram of an IS-136 compliant mobile device. A mobile's operation is carried out in processes that occur when it transitions from one state to another. The processes are carried out by software in the mobile. State 102 is the null state when the mobile is powered down. Upon being powered up by the power up action path 120, the mobile transitions to state 104 which is the Control Channel Scanning and Locking state. A mobile executes the DCCH Scanning and Locking procedure while in this state, attempting to find a digital control channel (DCCH). A mobile executes the Control Channel Selection procedure from the Control Channel Scanning and Locking state 104 once a DCCH is found using the DCCH Scanning and Locking procedure. The Control Channel Selection procedure is executed in order to determine if the DCCH is suitable for camping.
Communication between the mobile device and the base station employs a time division multiple access (TDMA) frame structure shown in FIG. 2, where each frame 200 and 200′ has six time slots. Frames 200 and 200′ shown in FIG. 1, occur at different frame times in the same 30-kHz RF channel. In the digital PCS full rate mode, each user is allowed access to two of the six time slots in a frame, so one RF channel can simultaneously serve up to three users. The frames have a duration of 40 ms and are organized into superframes of 16 TDMA frames, having a duration of 640 ms. A hyperframe consists of two superframes, and has a duration of 1.28 seconds. Frequency division multiple access (FDMA) is used to support more users by assigning multiple RF channels for use in the same cell. A mobile user transmits in the reverse or uplink direction on one frequency and receives in the forward or downlink direction on a different frequency, using frequency division duplexing (FDD).
Only one forward digital control channel (DCCH) consisting of two time slots 202 and 208 in one RF channel of FIG. 2, is required in the forward or downlink direction from the base station to the mobiles. Similarly, only one reverse digital control channel (DCCH) consisting of two time slots in another RF channel, is required in the reverse or uplink direction from the mobiles to the base station. The logical channels of the DCCH in the forward direction (from the base station to the mobile) are divided into broadcast channels and point-to-point channels, while the reverse DCCH is a point-to-point channel. The frame 200 shown in FIG. 2 carries the forward digital control channel (DCCH) containing control channel selection parameters for SCANINTERVAL in time slot 202 and DELAY VALUE in time slot 208. Time slots 204 and 210 carry traffic from the base station (BS) to mobile 20 and time slots 206 and 212 carry traffic from the base station (BS) to mobile 20′. The frame 200A shown in FIG. 2 carries the forward digital control channel (DCCH) containing the access parameter for MAX_RETRIES in time slot 214. Time slots 216 and 222 carry traffic from the base station (BS) to mobile 20 and time slots 218 and 224 carry traffic from the base station (BS) to mobile 20′. Time slot 220 carries other forward digital control channel (DCCH) broadcast channel messages or point-to-point control channel messages.
Once a suitable digital control channel (DCCH) is found, action path 121 causes the mobile to transition to the DCCH Camping state 106 in FIG. 1. The DCCH Camping state 106 is the normal state for the mobile while in service on a DCCH and not processing a call. Upon entering the DCCH Camping state, a mobile must read control information from the base station contained in the DCCH messages 140. This information includes control channel selection parameters used by the mobile in the reselection of a DCCH, including the SCANINTERVAL parameter and the DELAY parameter. This information also includes access parameters used by the mobile to access the base station, including the MAX_RETRIES parameter. The mobile periodically executes a number of processes and procedures while in the DCCH Camping state 106, including control channel reselection. A mobile remains in the DCCH Camping state 106 while it reselects from one DCCH to another. In the process reselection to select the best DCCH, the mobile periodically measures the received signal strength on the neighboring control channels from base stations in neighboring cells. The mobile then evaluates whether another base station's control channel would be better. If a better control channel is found, the mobile tunes to the new base station's control channel, synchronizes, and begins to monitor it for messages.
While in the DCCH Camping state 106, a mobile periodically measures the signal strength on the current DCCH and all control channels in the neighbor cell list. This measurement process is called the Control Channel Locking procedure, and is executed every SCANINTERVAL. The Control Channel Locking procedure is part of the Control Channel Reselection procedure. The mobile must wait for a delay interval timed by its camping state delay timer 106A, whose maximum delay value is set by the DELAY parameter, before the mobile can begin periodically measuring the signal strength. The DELAY parameter keeps the mobile from considering a neighboring control channel as a reselection candidate until the time delay has been met.
Once the DELAY interval has expired in camping state delay timer 106A, the mobile executes the Control Channel Reselection procedure to find a better control channel from which to obtain service. It typically invokes this procedure from the DCCH Camping state 106 over action path 132. In the Control Channel Locking procedure, the mobile measures and averages the signal strength on the current DCCH and all neighboring control channels. It does this periodically at the measurement interval determined by the value of SCANINTERVAL broadcast on the current DCCH. The period is timed by the camping state scan timer 106B, whose maximum duration value is set by the SCANINTERVAL parameter. The signal strength of the current DCCH is measured every SCANINTERVAL. The signal strength of neighboring control channels is measured either every SCANINTERVAL or every other SCANINTERVAL.
The mobile leaves the DCCH Camping state 106 in FIG. 1 to process transactions such as registration in the Registration Proceeding state 108, call origination in the Origination Proceeding state 112, or received call termination in the waiting for order state 110. A condition that precipitates leaving the DCCH Camping state 106 and returning to the Control Channel Scanning and Locking state 104 is losing service on the current DCCH, such as by a radio link failure on action path 130 and being unable to find another control channel through reselection. For further details, see the IS-136 Digital Cellular/PCS specification, section 6.3.3.4.1. A mobile stays in the DCCH Camping state 106 while it reselects from one DCCH to another.
The mobile transitions along action path 142 in FIG. 1 to the Registration Proceeding state 108 after sending a Registration message on a DCCH or in response to a forced registration request from the base station. Registration is the process whereby a mobile identifies itself to the base station and makes itself available for service. It remains in the Registration Proceeding state 108 for up to sixteen seconds while waiting for a response from the base station to its registration attempt. It returns from this state on path 142′ upon expiration of the sixteen second timer indicating that the time to wait has expired.
The mobile transitions along action path 122 in FIG. 1 to the Origination Proceeding state 112 after sending an origination message on a DCCH and while waiting for a response from the base station. The MAX_RETRIES parameter is the access parameter used by the mobile to attempt accessing the base station up to a maximum number of attempts. The MAX_RETRIES parameter sets the maximum retry count in the call origination retry counter 112A. The mobile leaves the Origination Proceeding state 112 when it receives a digital traffic designation from the base station, and moves to the assigned channel along path 124. If the mobile determines that the origination message was not received by the base station, it returns to the DCCH Camping state 106.
The mobile transitions along action path 146 in FIG. 1 to the Waiting For Order state 110 after sending a Page Response message on a DCCH and while waiting for a response from the base station. A page is a message sent by the base station to inform the mobile that a message is waiting, either a voice message, a short message service (SMS) message, or a fax message. The mobile remains in Waiting For Order state 110 for up to 6.4 seconds while waiting for a response from the base station. The mobile leaves the waiting for order state 110 when it receives a digital traffic designation from the base station, and moves to the assigned channel along path 126. It returns from the waiting for order state 110 along path 146′ upon expiration of the 6.4 second timer indicating the time has expired to wait between sending a Page Response message and receiving a response from the base station.
The mobile transitions along action path 144 in FIG. 1 to the Shared Secret Data (SSD) Update Proceeding state 114 if it has sent a Confirmation message in response to a Notification message indicating an SSD update. Shared Secret Data (SSD) is a 128-bit pattern stored in the mobile's memory, used to support authentication, voice privacy, and message encryption. It leaves the SSD Update Proceeding state 114 upon expiration of a twelve second timer indicating the time has expired to wait between successfully sending a Confirmation message and receiving a response from the base station.
Overlay Networks
The IS-136 Digital Cellular/PCS system can form hierarchical cell structures such as shown in FIG. 3, to increase the capacity of a cellular system by positioning smaller area adjunct cells or microcells 310 to operate within a system 300 of larger area primary cells or macrocells 301, 302, and 303. Such cells exist in a layered or overlay configuration that enables greater re-use of the RF spectrum and provides continuity of communication between the cell layers. An example is an indoor microcell system 310 operating within an outdoor macrocell system 300.
In an overlay configuration where a microcell wireless system 310 shares the same spectrum as the macrocell system 300 which is the owner of the RF spectrum, it is necessary for the microcell system 310 to monitor the activity of the macrocell system 300 on all RF channels. This involves periodic signal strength measurements that are made both by mobiles 20 and by the base stations 10 in the microcell wireless system 310. The microcell system 310 partitions all of the monitored RF channels into two sets: a first set of channels that are likely to be interference-free and a second set of noisy channels. A list is compiled of the interference-free channels in the first set as a pool of available channels that can be assigned to the microcell base stations and their registered mobiles. In the above-referenced copending U.S. patent application Ser. No. 09/401,408, filed Sep. 22, 1999, to Mathilde Benveniste, entitled “Self-Configurable Wireless Systems: Spectrum Monitoring In A Layered Configuration”, a mechanism is discussed to acquire such measurements on downlink channels by the mobiles 20, by deploying the mobiles registered to the microcell system 310.
What is needed is a method for signal strength measurements on uplink channels by the microcell base stations 10. However, the measurement of signal strength on uplink channels by the microcell base stations 10 must deal with a special set of problems when there are relatively few microcell base stations 10 in a region of a registered mobile station 20.
In a region where there is a high-density of microcell base stations 10 available with overlapping coverage, mobiles 20 registered to an idle microcell base station 10 can be induced to register to a neighboring microcell base station while measurements are taken by the idle base station 10. A powered-up mobile 20 registers when it enters the coverage area of a base station 10, or when it is first powered on, and it is placed on the list of registered mobiles. This enables the microcell base station 10 to page it if there is an incoming call. An inactive mobile 20 [i.e., a mobile not engaged in a call] does not need a traffic channel. It is assigned a traffic channel when a call starts.
When a microcell base station 10 is idle (that is, not serving any calls), it can retune to different channels and take measurements of several channels at once, thus offering an efficient mechanism for spectrum monitoring. This can be achieved by disabling the Digital Control Channel (DCCH), thus forcing the registered mobiles 20 to reselect another control channel.
However, in sparse systems where there are relatively few microcell base stations 10 in a region, it is not desirable to cause a registered mobile station 20 to undertake control channel reselection. The capability to stay on the current control channel is desirable for a microcell system 310 because control channel reselection in a sparse system will likely force mobiles 20 to register on the outside, macrocell cellular system 300. Since return to the original control channel is not immediate, a mobile 20 could end up placing a call on the outside, macrocell system 300 while the subscriber thinks he/she is being served by the microcell system 310.
Thus, the prior art is confronted with at least two problems. One is to avoid causing the inactive mobiles registered on the measuring microcell base station to miss their control channel and thus be forced to re-select a macrocell base station. The second problem is to reduce the probability that a call initiated by a mobile during an uplink spectrum-monitoring measurement by a microcell base station, will cause the transfer of the call to a macrocell base station.